1. Field of the Invention.
The present invention relates to a method and means for reducing power consumption in a coded railway track circuit. More particularly, the invention relates to a method and means for placing a coded railway track circuit apparatus into a reduced power standby mode during periods of low vehicle activity.
2. Description of the Prior Art.
In the art of railway signalling, traffic flow through signalled territory is typically directed by various signal aspects appearing on wayside indicators or cab signal units located on board the vehicles. The vehicle operators recognize each such aspect as indicating a particular operating condition allowed at that time. Typical practice is for the aspects to indicate prevailing speed conditions.
For operation of this signaling scheme, the track is typically divided into cascaded sections known as "blocks." These blocks, which may be generally as long as two to three miles, are electrically isolated from adjacent blocks typically utilizing interposing insulated joints. When a block is unoccupied, track circuit apparatus connected at each end are able to transmit signals back and forth through the rails within the block. Such signals may be coded to contain control data enhancing the signalling operation. Track circuits operating in this manner are referred to as "coded track circuits." One such coded track circuit is illustrated in U.S. Pat. No. 4,619,425, issued Oct. 28, 1986 to Nagel. When a block is occupied by a railway vehicle, shunt paths are created across the rails by the vehicle wheel and axle sets. While this interrupts the flow of information between respective ends of the block, the presence of the vehicle can be positively detected.
Generally, coded track circuit apparatus can be functionally categorized into two types depending on their location within the signalled territory. The first type are end units, which have a separate communication link to the railway dispatching office or other central vehicle control location. These units are often placed at industrial sidings or highway crossings and are thus convenient to commercial power hookup. The second type are intermediate units which are connected to rails in adjacent blocks, thus coupling information around the insulated joints. In this way, ultimate communication between end units is facilitated. Often, these intermediate units are located in remote areas. Powering these intermediate units has often required installation of lengthy and expensive stretches of buried or pole-mounted cable.
The need to install power cables to intermediate units can be eliminated in some areas using self-contained battery systems which may be charged by solar panels. Present intermediate units, however, have consumed power at a rate requiring such battery systems to have significant capacity. Since the cost of these battery systems is directly related to power capacity, a significant disincentive has existed for their use. Even when power cables are run to the intermediate units, storage batteries are required at each location to provide backup power in the event of commercial power failure. The size and cost of these batteries also depend directly on average power consumption.